Wake turbulence occurs when the lift necessary for flying is generated. In a wing which generates lift, the pressure on the underside exceeds the pressure on its top. Consequently, at the ends of the wing, air flows from the underside to the top, as a result of which a strong vortex is generated, namely the so called “airfoil vortex”. Furthermore, at the trailing edge of the wing the fluid layers coming from the top and from the underside flow past each other in different directions so that a free shear layer arises, which in the direction of the wing span is connected to the respective airfoil vortex. This free shear layer together with the respective airfoil vortex rolls itself into two individual vortices that may rotate in opposite directions in the manner of small horizontal tornadoes which, depending on the size of the aircraft, at their centres can reach top speeds of up to 360 km/h.
Wake turbulence is evident for the equivalent of several hundred wingspans downstream before it decays. This means that in the case of large aircraft such wake turbulence can have a life of several minutes and a length of up to 30 km.
This could have grave consequences in particular for a small aircraft that flies into wake turbulence; for example severe local fluctuations in aerodynamic forces could lead all the way to loss of a stable flight position.
For this reason, aircraft should have to maintain a corresponding distance to each other, in particular during takeoff and landing. The airfoil vortices generated by the wing, which vortices result from the continuous rolling action, thus endanger aircraft that follow, and are a limiting factor in determining takeoff and landing frequencies.